Abstract

The two-stage, fully differential audio amplifier with 0.18μm CMOS technology and TSMC process is proposed. Telescopic amplifier created stage one and common emitter structure used to provide high gain in stage two. The amplifier has a ±1.2V differential output swing and a gain greater than 127dB while consuming power less than 10mW from 1.8V supply voltage. The step response of the audio amplifier is about 0.04us and the gain-bandwidth of the circuit is near 1.1GHz. Hspice software used to produce simulation results.

1. Introduction

With decreasing channel length, CMOS technology becomes an attractive candidate for integrated circuits [1]. Usually the operational amplifier is the block with the highest power consumption in analog integrated circuits in many applications. Low power consumption in circuits is becoming more important in handset devices, so it is a challenge to design a low power amplifier. There is a trade off among speed, power and gain for an amplifier design because usually these parameters are called contradicting parameters [2]. Usually three kinds of operational amplifier exist: two-stage, folded-cascode and telescopic. The telescopic amplifier consumes the least power as compared with the other two amplifiers, so it is widely use in low-power consumption applications [3, 4]. A lower current (which follows in differential pair transistors) can improve small-signal resistance of transistors and increase the amplifier’s gain. Recently, telescopic amplifier design and research focused on improving gain and output swing [5, 6]. One of the telescopic amplifier disadvantages is severely limited output swing. In this paper high-gain and low-power fully differential audio amplifier based on telescopic and common emitter structure is proposed. The circuit has a high output swing and gain-bandwidth. Hspice software and 0.18μm CMOS technology and TSMC process used to produce simulation results. This circuit is suitable for on-chip integrated audio amplifier in the communication system.

2. Design of Two-stage Amplifier

Figure 1 shows the complete circuit of the two-stage fully differential audio amplifier. The NMOS telescopic amplifier transistors (M1-M4) have the same characteristics such as w, l, AD, AS, PD, PS and … (This is also applied for PMOS (M5-M8) transistors). The emitters of M7 and M8 are connected to VDD. Eq. (1) describes the maximum allowable current consumes from a 1.8V supply voltage of the amplifier.

(1)

(2)

(3)

(4)

It depends on designer to assign these currents. Then the designer must obtain the telescopic amplifier transistor’s dimensions. (In equations VOD, Cox, VT and μ are:

By assigning suitable voltages to VOD1 and |VOD5|, telescopic amplifier transistor’s dimensions can be calculated. Usually in Eq. (11) K'n is not equal to K'p (). The designers should notice this issue, when they consider the values for overdrive (VOD) voltages (W: MOSFET channel width, L: MOSFET channel length).

(11)

Because of using the same characteristics for M1-M4 and M5-M8:

(12)

(13)

(14)

(15)

For calculating MOSFET’s dimensions such as:

1-AD: Drain diffusion area

2-AS: Source diffusion area

3-PD: Perimeter of the drain junction

4-PS: perimeter of the source junction

Eq. (16) and (17) used.

(16)

(17)

Ls (in Eq. (16) and (17) ) depends on technology, for example for 0.18μm COMS technology Ls is 0.48μm or for 0.13μm CMOS technology it is 0.55μm. M9-M11 used to bias circuit. Note that all of the transistors should be biased in the saturation region. To achieve this issue the designer should consider saturation’s conditions. Solving Eq. (11) requires K' initial value. For finding this value, two methods suggested:

First- Simulate a simple circuit includes a transistor with the same amplifier transistor’s characteristics and using simulation results for finding K'.

Table 3. MOSFET’s parameter’s Simulation Results

Figure 2 represents one-sided output signals simulation result (out_1 and out_2) which is compatible with Eq. (22). Step response is the time behavior of the outputs of a general system when its inputs change from zero to one in a very short time. The concept can be extended to the abstract mathematical notion of a dynamical system using an evolution parameter. From a practical standpoint, knowing how the system responds to a sudden input is important because large and possibly fast deviations from the long term steady state may have extreme effects on the component itself and on other portions of the overall system dependent on this component. In addition, the overall system cannot act until the component's output settles down to some vicinity of its final state, delaying the overall system response. Formally, knowing the step response of a dynamical system gives information on the stability of such a system, and on its ability to reach one stationary state when starting from another. The step response of the fully differential audio amplifier is displayed in Figure 3. Figure 4 describes the structure used for simulating the step response of the audio amplifier. Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input. In simplest terms, if a sine wave is injected into a system at a given frequency, a linear system will respond at that same frequency with a certain magnitude and a certain phase angle relative to the input. Two applications of frequency response analysis are related but have different objectives. For an audio system, the objective may be to reproduce the input signal with no distortion. That would require a uniform (flat) magnitude of response up to the bandwidth limitation of the system, with the signal delayed by precisely the same amount of time at all frequencies. That amount of time could be seconds, or weeks or months in the case of recorded media. The frequency response and gain-bandwidth value of the fully differential audio amplifier are shown in Figure 5. AC simulation results include:

5. Conclusion

A two-stage fully differential audio amplifier has been successfully designed and simulated. With this method and process, we achieved a gain greater than 127dB and the output swing about 2.4V from 1uV sinusoidal input voltage. The circuit consumes power less than 10mW from 1.8V supply voltage. The step response of the audio amplifier is about 0.04us that is suitable for on-chip amplifier in microphone. The gain-bandwidth of the audio amplifier is near a 1.1GHz and the frequency response curve shows that audio signals with 20kHz bandwidth can be amplified with high gain. The proposed structure is a suitable for low-power, small area occupation and high gain on-chip integrated audio amplifier in communication system.

Acknowledgement

The authors would like to express their sincere thanks to Pooya Torkzadeh for his valuable comments.